Various improvements to Clang's code-completion infrastructure:

  - Introduce more code-completion string "chunk" kinds that describe
  symbols, the actual text that the user is expected to type, etc.
  - Make the generation of macro results optional, since it can be
  slow
  - Make code-completion accessible through the C API, marshalling the
  code-completion results through a temporary file (ick) to maintain
  process separation.

The last doesn't have tests yet.

llvm-svn: 86306